Electromagnetic waves sent at terahertz frequencies, known as terahertz radiation, terahertz waves (THz) rays, occur in the region of the electromagnetic spectrum between 300 gigahertz (3×1011 Hz) and 3 terahertz (3×1012 Hz), and correspond to the wavelength range of between 1 millimeter (high-frequency edge of the microwave band) and 100 micrometer (long-wavelength edge of far-infrared light).
THz-rays, which are shorter than microwaves and longer than infrared, have potential usage in biomedical and security applications in that THz-rays are safe, and non-ionizing, and can pass through such materials as clothing, paper, cardboard, wood, masonry, plastic, ceramics, as well as penetrate fog and clouds. THz radiation is safe for biological tissues (unlike X-rays), and images can be formed with terahertz radiation having resolution of less than 1 mm. THz radiation has potential spectroscopic uses in that while many materials are transparent to THz, many materials exhibit unique spectral identifiers when exposed to terahertz radiation, including explosives, pharmaceuticals, and illegal narcotic substances. Accordingly, items of interest can be observed through normally visually opaque intervening layers, such as packaging and clothing. To date, THz rays have not yet been widely used, partly, because of the difficulty in producing them at sufficient intensity. For example, in the journal article entitled, “Generation of Terahertz Radiation with Two Color Semiconductor Lasers,” Hoffman and Hoffman, Laser and Photonics Reviews, Rev. 1, No. 1, 44-56 (available on-line Feb. 13, 2007) (hereinafter Hoffman & Hoffman) though time-domain THz spectroscopy has been shown to be extremely powerful, it suffers from its enormous price (at least $200,000) and complexity, which restricts the use of the system.